It is known that an internal combustion engine generally includes an engine block defining one or more cylinders, each of which accommodates a reciprocating piston coupled to rotate a crankshaft. A cylinder head cooperates with each of the pistons to define a number of combustion chambers, where a fuel and air mixture is injected once per engine cycle and ignited, resulting in hot expanding exhaust gasses that cause reciprocal movement of the pistons. The fuel is provided in each of the combustion chambers by a dedicated fuel injector, which receives the fuel at high pressure from a fuel rail in fluid communication with a high pressure fuel pump that increase the pressure of the fuel received from a fuel source.
Conventionally, each fuel injector provides the fuel into the combustion chamber by performing a plurality of injection pulses per engine cycle, according to a multi-injection pattern. This multi-injection pattern usually includes a main injection, which is generally executed just before the Top Dead Center (TDC) of the piston to generate torque at the crankshaft, and several smaller injections, which may be executed before the main injection (e.g. pilot-injections and pre-injections) and/or after the main injection (e.g. after-injections and post-injections). Each of these small injection pulses is made to inject into the combustion chamber a small quantity of fuel, typically lower than 2.5 mm3 (for example 1 mm3), with the aim of reducing polluting emissions and/or combustion noise of the internal combustion engine.
The fuel injectors are essentially embodied as electromechanical valves having a needle, which is normally biased in a closed position by a spring, and an electro-magnetic actuator (e.g. solenoid), which moves the needle towards an open position in response of an energizing electrical current. The energizing electrical current is provided by an electronic control unit, which is generally configured to determine the fuel quantity to be injected during a single injection pulse, to calculate the duration of the energizing electrical current (i.e. the energizing time) necessary for injecting the desired fuel quantity, and finally to energize the fuel injector accordingly.
However, it may happen that the fuel quantity actually injected during an injection pulse is different from the desired one. This undesirable condition may be caused by several reasons, including drift of the injection characteristics and production spread of the fuel injectors. In particular, the correlation between the electrical command and the injector needle displacement can be effected by factors hard to be controlled during the injectors manufacturing, such as magnetic permeability drift of the actuator, tolerance of the needle spring coefficient, aging effect, and temperature dependency. Therefore, it is very likely that two fuel injectors (even of the same production slot) behave differently in response of the same electrical command.
As a result of all these factors, for a given energizing time at a given fuel rail pressure, the fuel quantity actually injected into the combustion chambers of an internal combustion engine may be different injector-by-injector and/or vary with the aging of the injection system. This problem is particularly critical for the small injection pulses, whose good precision and repetitiveness is essential in order to achieve the expected improvements in terms of polluting emission and combustion noise.
To solve this drawback, when the internal combustion engine running in a cut-off condition, the electronic control unit is configured to perform a learning phase of the actual fuel injected quantity. The learning phase provides for commanding a fuel injector at the time to perform several small injection pulses in a sequence of engine cycles, for detecting in some way the fuel quantity actually injected during these small injection pulses, and then for determining a correction to be applied to the energizing time in order to minimize the difference between the desired and the detected fuel injected quantities. Such learning tests are repeated at predetermined time intervals and performed for each fuel injector of the engine individually.
According to the known solutions, the fuel quantity actually injected may be estimated on the basis of input signals deriving from different kinds of sensors such as knock sensors or on the basis of the crankshaft wheel signal. The major drawback of these prior solutions lies in the fact that such fuel quantity estimations are indirect. Therefore, these signals, for example the crankshaft wheel signal or other signals, are easily effected by noise and other disturbances coming from external environment such as rough roads, electric loads or other external or internal conditions, so that the resulting estimation may be not always reliable. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.